1. Field of the Invention
The present invention relates to a semiconductor position sensor.
2. Related Background Art
Position sensitive detectors (PSDs) are known as devices for measuring the distance to an object to be measured, by the principle of so-called triangulation or the like. The PSDs are mounted as distance-measuring devices of the active type on imaging devices, and in such imaging devices focusing of a taking lens is carried out based on the distance to the object, measured by PSD.
The PSDs can provide continuous output according to positions of incidence light, but their S/N ratios become heavily lowered under circumstances of low intensity of signal light, e.g., in measurement of far range. For this reason, it is impossible to improve the ranging accuracy for measurement in the far range in the distance-measuring systems using the PSDs. In contrast with it, multiple-part split photodiodes such as two-part split photodiodes (PDs) are unable to provide continuous output according to positions of incidence, different from the PSDS, but are able to demonstrate higher detection sensitivity, because there occurs little deterioration of S/N ratios even with a low intensity of signal light.
Then, Japanese Patent Application Laid-Open No. H04-313279 describes a semiconductor optical position sensor in the form of combination of PSD with two-part split PD.
FIG. 14 is a top view of such a semiconductor position sensor. This position sensor is provided with the PSD consisting of a layer having a semiconductor-conductive-strip-forming region 20 provided on a semiconductor substrate 1 and a pair of signal output electrodes 5 provided at two ends of the region 20, and with the two-part split PD 21 located on the far range side where the intensity of signal light is low. In this position sensor, the PSD provides continuous output according to positions of incidence of signal light, while the two-part split PD performs position detection with good sensitivity at the positions where the signal light is weak.
In this structure, however, the chip area is large and, because light impinges on the PSD and a separation portion of the two-part split PD, only part of the signal light is utilized for the position detection and it is thus impossible to enhance the detection sensitivity where the spot size of the illuminating signal light is small. The present invention has been accomplished in view of this problem and an object thereof is to provide a semiconductor position sensor that is smaller than those of the conventional technology and that can achieve improvement in the detection sensitivity.
In order to solve the above problem, a semiconductor position sensor according to the present invention is a semiconductor position sensor in which electric currents outputted from two ends of a semiconductor electroconductive area vary according to positions of incident light on a photosensitive area, wherein gate means capable of interrupting electric conduction between the two ends is provided on an intermediate area of the layer of the semiconductor conductive area. The semiconductor position sensor functions as a Psd when the gate means enables electric conduction between the two ends of the semiconductor conductive area, but functions as a multiple-part split or single PD when the gate means interrupts the electric conduction between them.
It is preferable that the gate means comprise an insulating film laid on the semiconductor, and a gate electrode laid on the insulating film. A channel formed immediately below the gate electrode through the insulating film is controlled by a voltage applied to the gate electrode. This channel disappears when the voltage of the gate electrode reaches a predetermined value. Then the electric conduction is interrupted between the two ends of the layer of the semiconductor conductive area.
In order to efficiently collect charge generated according to incidence light to the photosensitive area, toward the semiconductor conductive area, it is preferable that the semiconductor position sensor of the present invention comprises a layer having a plurality of electroconductive branching strips extending from the semiconductor conductive strips.
For reducing influence of the branching strips on the detection accuracy, it is desirable to set an impurity concentration thereof high, so as to decrease the resistivity. However, if the resistivity of the branching strips is substantially equal to that of the semiconductor conductive area, they can be fabricated in the same step, whereby the production time can be reduced.
In the semiconductor position sensor of the present invention, it is preferable that the semiconductor conductive strip immediately below the gate electrode have an impurity concentration different from that of the two ends, whereby the impurity concentration of this area permits one to set the gate voltage necessary for generation of the channel.
The gate means may comprise an insulating film laid on the semiconductor conductive strip, and a plurality of gate electrodes laid each on the insulating film. In this case, the ratio of areas of respective photodiode zones of the multi-part split photodiode can be made variable.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.